Conventional interferometers arranged for comparing object and reference surfaces using the mechanism of interference generally require illuminating and imaging systems that are matched in size to the size of the object surface. Typically, the illuminating systems collimate the object and reference beams at a common diameter encompassing the desired measurement area of the object surface. Imaging systems collect the collimated object and reference beams following respective reflections from object and reference surfaces.
Generally, the sizes of the object and reference beams are not significantly affected by their encounters with (e.g., reflections from) the object and reference surfaces. If the object and reference beams are produced by the illuminating system as expanding beams, even larger diameter imaging optics are required to collect the object and imaging beams following their encounters with the object and reference surfaces because the beams continue to expand following their encounters. The illuminating systems generally employ expensive collimating optics so that individual rays of the object and reference beams approach the object and reference surfaces at normal incidence or at least at a constant angle of incidence.
The measurements carried out by conventional interferometers compare object surfaces to reference surfaces that share the same nominal geometric form as the object surfaces. Differences between the object and reference surfaces measured as path length variations between the object and reference beams are generally attributed to errors in the object surface. Accordingly, the reference surfaces are generally made as accurately as possible. Most reference surfaces are highly polished mirrors that exhibit spectral reflection.
The unambiguous measurement interval of conventional interferometers is related to the central wavelength (frequency) of the object and reference beams. Interference patterns produced by overlapping images of the object and reference surfaces contain fringe patterns of constructive and destructive interference between the object and reference beams. A single cycle of constructive and destructive interference between adjacent fringes is the unambiguous measurement interval. Fringes produced by reflective surfaces at normal incidence in a single pass have fringe spacings representing surface height variations of the object surface equal to one-half of the central wavelength of the interfering beams. Given the usual range of optically transmissive wavelengths, this has two main effects. First, very smooth (e.g., specular) reference surfaces are required. Second, only object surfaces having limited roughness can be unambiguously measured.
Multiple wavelength interferometers can be used for expanding the unambiguous measurement interval of conventional interferometers. Broad-band interferometers, also referred to as “white light” interferometers, expand the measurement interval by measuring a succession of physical displacements between object and reference surfaces required to identify points of peak interference contrast that occur at equal optical path length differences between the object and reference surfaces. The amounts of displacement required to position each point on the object surface at an equal optical path length with a corresponding point on the reference surface are measured to map the surface topology of the object surface.
Frequency-scanning interferometers, also referred to as wavelength-scanning interferometers or multi-wavelength interferometers, derive their broader range of measurement from a plurality of interference patterns produced at a succession of different beam frequencies (or wavelengths). In contrast to conventional interferometers that compare path length differences between points within the same interference patterns and use additional interference patterns to make finer measures or to resolve ambiguities within the unambiguous measurement interval, frequency-scanning interferometers can measure points common to the plurality of interference patterns individually, based upon interferometric (e.g., intensity or phase) fluctuations of the corresponding points within the plurality of interference patterns produced at different beam frequencies.
As such, a wider range of surface roughness can be unambiguously measured by frequency-scanning interferometers. Conventional interferometers are typically limited to measuring step sizes in the direction of illumination within the fringe spacing of their interference patterns, which relate directly to the wavelength of the illumination. The measurement of such step sizes by frequency-scanning interferometers is normally independent of the nominal wavelength of illumination, depending instead on the average interval between the beam frequencies. The finer the interval, the larger the range of unambiguous measurement. Thus, frequency-scanning interferometers can provide measures of rough or diffuse object surfaces at beam frequencies that produce speckle-ridden interference patterns unintelligible to conventional interferometers.
Frequency-scanning interferometers are especially useful for measuring surface profiles (topographies) of test objects as measures of surface variations taken normal to a reference plane or surface. Recent developments of frequency-scanning interferometry include the use of components such as tunable diode lasers and CCD detector arrays. As a result, compact, accurate, and fast systems have been developed, which have the capability of performing measurements for a wide range of test surfaces.
A known frequency-scanning interferometer system 10 is depicted in FIG. 1. While in the overall form of a Twyman-Green interferometer, a tunable laser 12 under the control of a computer 14 produces a measuring beam 16 that can be tuned through a range of different frequencies. An illuminating system including beam conditioning optics 18 expand and collimate the measuring beam 16. A folding mirror 20 directs the measuring beam 16 to a beamsplitter 22 that divides the measuring beam 16 into a object beam 24 and a reference beam 26. The object beam 24 retroreflects from a test object 30, and the reference beam 26 retroreflects from a reference mirror 32. The beamsplitter 22 recombines the object beam 24 and the reference beam 26, and imaging optics 34 (such as a lens or group of lenses) of an imaging system focus overlapping images of the test object 30 and the reference mirror 32 onto a detector array 36 (such as a CCD array of elements). The detector array 36 records the interferometric values of an interference pattern produced by path length variations between the object and reference beams 24 and 26. Outputs from the detector array 36 are stored and processed in the computer 14.
The elements (pixels) of the detector array 36 record local interferometric values subject to the interference between the object and reference beams 24 and 26. Each of the interferometric values is traceable to a spot on the test object 30. However, instead of comparing interferometric values between the array elements (pixels) to determine phase differences between the object and reference beams 24 and 26 throughout an interference pattern as a primary measure of surface variation, a set of additional interference patterns is recorded for a series of different beam frequencies (or wavelengths) of the measuring beam 16. The tunable laser 12 is stepped through a succession of incrementally varying beam frequencies, and the detector array 36 records the corresponding interference patterns. Data frames recording individual interference patterns numbering 16 or 32 frames are typical.
The local interferometric values vary in a sinusoidal manner with the changes in beam frequency, cycling between conditions of constructive and destructive interference. The rate of interferometric variation, e.g., the frequency of intensity variation, is a function of the path length differences between the local portions of the object and reference beams 24 and 26. Gradual changes in intensity (lower interference frequency variation) occur at small path length differences, and more rapid changes in intensity (higher interference frequency variation) occur at large path length differences.
Discrete Fourier transforms can be used within the computer 14 to identify the interference frequencies of interferometric (e.g., intensity) variation accompanying the incremental changes in the beam frequency of the measuring beam 16. The computer 14 also converts the interference frequencies of interferometric variation into measures of local path length differences between the object and reference beams 24 and 26, which can be used to construct a three-dimensional image of the test object 30 as measures of profile differences from a surface of the reference mirror 32. Since the reference mirror 32 is planar, the determined optical path differences are equivalent to deviations of the object 30 from a plane. The resulting three-dimensional topographical information can be further processed to measure important characteristics of the object 30 (e.g. flatness or parallelism), which are useful for quality control of precision manufactured parts.
Although frequency-scanning interferometers can be used to measure object surfaces having a much greater range of surface roughness than can be measured by conventional interferometers, the dimensions of the illuminating and imaging optics remain much the same. For example, the beam conditioning optics 18 are sized to expand and collimate the measuring beam 16 so that the object beam 24 and the reference beam 26 encompass similarly sized areas of the test object 30 and the reference mirror 32. The imaging optics 34 are similarly dimensioned to focus the reflected but still collimated object and reference beams 24 and 26 onto the detector array 36. Thus, the size of the illuminating and imaging systems is matched to the size of the test object 30 and reference mirror 32. The cost of such optical systems becomes prohibitively expensive for measuring large test objects and can remain considerable even for measuring small test objects. Collimating optics, especially those sized for measuring large test objects are particularly expensive and occupy considerable space, which are detrimental to making more compact interferometer systems.